AC/DC switching power supplies, such as laptop computer chargers, must conform to certain standards. One of these standards is the power factor (PF), which requires conditioning of the input current from the external AC outlet. The standard AC outlet is typically designed to handle 10 A current at 110 V, so at 1.1 kW (=10 A×110V) power, the standard specifies the PF must be close to 1.0 or 100%. At a power level of 75 W, the standard specifies that the PF must be close to 0.8. However, previously, standard power supply without active power factor correction (PFC) stage can only achieve a 0.5 to 0.6 PF at a power level of 75 W. Therefore, prior art utilizes a double stage AC/DC switching power supply with power factor correction (PFC) to achieve the higher PF with good output power regulation. In a double stage topology, a first stage, which is most commonly a boost stage, takes care of the input current conditioning and energy storage for dealing with the sinusoidal characteristic of the AC (alternating current) outlet, while a second stage, which is an isolated DC/DC converter, takes care of the output voltage regulation. A typical double stage topology is shown in FIG. 1. In FIG. 1, the first stage converts AC input to a loosely regulated 400V intermediate DC bus with power factor correction (PFC). The second stage, a front-end DC/DC converter, will convert 400V DC into a tightly regulated 20V DC bus. The second stage is, in many recent applications, an LLC resonant converter. The double stage solution has several issues, most important being that the power is processed twice via serial/cascade connection of the two stages and that the boost stage has to be able to process twice the average output power on the peak of the input AC line. The resulting designs are therefore complex, expensive and with reduced power density.
Single stage AC/DC topology can be designed to overcome the complexity of the two stage solution, but most single stage topologies fall short of providing all the advantages of the two stage solution. One of the most popular approaches is the single stage PFC (power factor correction) Fly-back, which can provide a good power factor (PF) but has a very poor output voltage regulation (i.e., cannot reject the AC line ripple) and a very poor load transient response. The PFC Fly-back approach can have an improved solution, but the improved solution still lacks the energy storage capability of a two stage topology.
For relatively low power (i.e., 100 W or less), the input current conditioning can be relaxed because the harmonic currents limit can be easier met, the requirements being tailored for 1 kW or more of power from the AC outlet. However, such approaches of single stage PFC AC/DC topologies with storage capability have increased complexity and issues with the control of the storage element voltage.
The symmetrical LLC resonant topology is one of the promising solutions for the DC/DC stage of the two stage converters. Its characteristic frequency control is a trade-off to the soft-switching and low harmonic current content that it provides. However, its main limiting factor is the narrow input voltage range for optimal operation. An alternative approach is to use a constant frequency LLC resonant topology, where the output voltage control is obtain by asymmetric drive of the LLC tank.
Therefore, what is desired is a single stage topology where energy storage is produced in a controlled manner and regulated output voltage is delivered with reasonable well-conditioned input current.